In summary, a monolithic integrated circuit involves a plurality of transistors formed in a silicon substrate. The substrate is coated with dielectric and the transistors are interconnected by means of a metalization pattern on the dielectric coating that contacts transistor regions through windows in the dielectric coating. A second dielectric coating and second metalization pattern can provide a second layer of interconnections. In integrated circuits made from a plurality of insulated gate transistors, a second level of metalization can be obtained without applying a second dielectric coating and second matalization pattern. Instead, a conductive interconnection pattern is formed from portions of the polycrystalline silicon coating that is used to form the gate electrodes on the insulated gate transistors in the circuit. This supplementary interconnection is referred to as a buried contact type of interconnection. It is achieved by opening a contact window in an active device area before depositing the blanket polycrystalline silicon coating from which the gate electrodes are formed. Diffusion of the dopant from the polycrystalline silicon into the underlying substrate forms a low resistance contact with the substrate. Such contacts, i.e. buried contacts can be used to not only interconnect transistors but also to interconnect a transistor gate region with its own source region.
I have now found how to dispose a buried contact on an active device area, and add to it sourcedrain doping from another active area, to form a new component in an insulated gate transistor type integrated circuit. More specifically, I have found that in an N-type well complementary insulated gate (CMOS) integrated circuit, one can produce a bipolar lateral transistor in that circuit without adding steps to the existing process for making the integrated circuit without the bipolar transistor.